After the visual array is displaced by means of prism spectacles human subjects initially misreach when attempting to grasp external objects. With continued exposure to the visual rearrangement accurate sensory-motor coordination is soon reestablished. The nature of this adaptation as well as the conditions that must be met before it occurs are poorly understood. Nevertheless, it has been shown repeatedly that active movements made during exposure to visual rearrangement are more effective than passive movements of similar magnitude and frequency in leading to adaptation. The experiments proposed are designed to determine 1) whether during visual rearrangement the discordance between the visual direction and the proprioceptively-specified direction of the hand is the factor that leads to adaptation, 2) whether the greater effectiveness of active movements during exposure results from a greater accuracy of position-sense information during voluntary movement.